Functional Programming (FP) styles are generally preferred to Object Orientated (OO). FP programs have two major advantages: they are much easier to test and they scale much better.

Even if you are programming as an OO programmer you will be a better programmer if you know FP

Basics

• Single Responsibility Functions
• Immutable State
• Focus on how to cleanly compose functions, and how to test these compositions
• Learn how to use the builtin higher order functions.
• Functions should avoid side effects, and there should be clean separation of pure and impure functions
• Prefer protocols/types classes to inheritance

Unlike object orientated guidelines, I was unable to find an equivalent of SOLID. The following are slightly opinionated (and their reasoning explained). As these are opinionated, in a project you can take different guidelines, but please be aware of the reasons

Advanced topics

• DSLs are encouraged
• Avoid Monadic Transformers
• Prefer Kleisli’s to Monads